Polycarbonate resins for use as optical material is preferably free from particles as well as from impurities, e.g., halogen components, which may be incorporated during production.
In the conventional processes for producing an aromatic polycarbonate resin molding material from a polymerization mixture obtained by interfacial polymerization of a bisphenol and phosgene in the presence of a halogenated hydrocarbon as a good solvent, a resin solution is separated from the polymerization mixture and, after purification, the resin is recovered from the solution, dried, and pelletized by extrusion; or the resin solution is property concentrated and then extruded while removing the solvent by distillation to obtain pellets.
Methods for separating the resin from the purified resin solution include (a) a concentration method which comprises concentrating a solution of the resin in a good solvent which may contain a poor solvent in such a proportion that does not cause precipitation to thereby gelatinize the resin (called "gelation by concentration method", "gelation by solvent distillation method", "gelation by flash concentration method", etc.) or which comprises dropwise adding the above-described resin solution to warm water to distill the solvent to thereby gelatinize the resin (called "dropping in warm water method") and (b) a precipitation method which comprises dropwise adding the above-described resin solution into a poor solvent or vice versa.
However, these conventional separation methods involve disadvantages such that the solvent remaining in the resulting resin gel is difficult to remove by drying depending on the kind of the resin gel and the permissible residual amount of the solvent and that a large-sized drying apparatus is required. Further, since the drying and extrusion steps necessarily handle a solid powder having been dried to a high degree, it is unavoidable that the polycarbonate resin undergoes denaturation such as gelation or carbonization or causes wear of equipments due to mechanical abrasion. As a result, it has been difficult to inhibit an increase of particle content. In more detail, the increase of particle content has hitherto been coped with by adopting methods or conditions for obtaining a wet powder which may reduce the load of drying (for example, a combination of the dropping-in-warm-water technique and slurry grinding) and controlling the shape or structure of the resulting wet powder to obtain a spherical and porous resin powder which can be dried with comparative ease and is less causative of mechanical wear, or by strictly selecting an apparatus for extrusion or minutely controlling drying or extrusion conditions. Nevertheless, it has been still difficult to completely inhibit the increase of the particle content during the drying or extrusion step. Although direct extrusion of a concentrated polycarbonate resin solution to obtain pellets is advantageous from the standpoint of inhibition of the particle content increase, it is difficult to reduce methylene chloride, a solvent for the polycarbonate resin, to such an extent causing no substantial problem.
On the other hand, methods for purifying the aromatic polycarbonate resin solution include a method comprising repeatedly washing the resin solution with pure water in a batch system and separating the resin solution layer from the aqueous layer by allowing the system to stand or by centrifuging at the point when the aqueous layer becomes neutral, a method comprising contacting the polycarbonate resin solution with activated carbon to remove monomers and oligomers as disclosed in JP-B-41-8475 (the term "JP-B" as used herein means an "examined published Japanese patent application"), a method comprising passing a mixed solution of the polycarbonate resin solution and water through a hydrophilic filter layer having a water contact angle of 40.degree. or less (e.g., cotton cloth, filter paper, diatomaceous earth, and glass cloth) to separate into an aqueous phase and a resin solution phase from which water-soluble impurities have been removed as disclosed in JP-B-46-41622.
Since mere washing with pure water is not sufficient for completely remove free chlorine, it is necessary to decrease the water content in the resin solution. However, reduction of the water content in the resin solution by centrifugal separation to 0.2% by weight or less essentially requires an extraordinarily large force and time for centrifuging and, thus, the apparatus therefor should be large-sized, making it difficult to utilize this technique on an industrial scale. The method utilizing a filter layer finds difficulty in reducing the water content of the resin solution to 1% by weight or less. Therefore, the resin solution obtained by these conventional methods necessarily contains about 0.5 ppm of free chlorine. As a result, it has been difficult to prevent damages of a metal layer for recording film due to spot or overall corrosion under high temperature and high humidity conditions.
In order to solve this problem, it has been proposed to take up free chlorine and convert it to a stable and harmless substance instead of removal of free chlorine. This concept has been embodied typically by incorporating a known stabilizer for polycarbonate. However, none o the state-of-the-art stabilizers is satisfactory.